1. Field of the Invention
The present invention relates to a rechargeable battery device having an overcharge protection function and an overdischarge protection function for a rechargeable battery and having a simple structure in which safety is ensured.
2. Description of the Related Art
A rechargeable battery, such as a Lithium-ion battery and a Nickel-Metalhalide battery, is deteriorated in battery characteristics by overcharge. Overdischarge of the rechargeable battery has the possibility of decreasing the safety of the battery. Likewise, if the discharge current of the rechargeable battery exceeds its rated value, the safety is possibly decreased by abnormal heat generation of the battery.
Given these factors, for example, as disclosed in Unexamined Japanese Patent Publication No. 2001-126772, a semiconductor switch element (for example, MOS-FET) that electrically continues or interrupts a charging/discharging line of a rechargeable battery is conventionally interposed in the charging/discharging line in series. When the charge voltage of the rechargeable battery exceeds overcharge protection voltage or when the discharge current of the battery exceeds rated current, the semiconductor switch element is turned off so that the charge and discharge are forbidden. As a countermeasure against a failure of the semiconductor switch element, the charging/discharging line is further interposed with a thermo-fuse (nonreturn-type switch) in series as disclosed, for example, in Unexamined Japanese Patent Publication No. 2001-176561. The charging/discharging line is interrupted by blowing out the thermo-fuse in the event of a failure of the semiconductor switch element.
When overcharge of the rechargeable battery is detected, however, if the thermo-fuse interposed in the charging/discharging line is operated, the electric energy stored in the rechargeable battery cannot be recovered through the charging/discharging line. There is another idea that charging and discharging lines for the rechargeable battery are arranged independently from each other, and that each of the lines is provided with the semiconductor switch element and the thermo-fuse. The fact is, however, that a high-current thermo-fuse (nonreturn-type switch) has not been created, which is suitable for the discharge current to be supplied to load with large rated current, such as a machine tool and a battery-assisted bicycle.
Conventionally, the charge voltage of the rechargeable battery and the charge and discharge currents of the battery are detected, and the temperature of the semiconductor switch element is monitored. When a malfunction of the rechargeable battery or of the semiconductor switch element is detected from the monitoring information, a malfunction detection signal is transmitted through communication means to the load provided with a charger or the like, and the charge and discharge of the rechargeable battery are forbidden at the load (charger) side. If such a countermeasure is taken, the rechargeable battery device becomes especially complicated in structure, so that it is undeniable that the battery costs high.